Method for the preparation of diborane



United States Patent 3,135,583 METHOD FOR THE PREPARATION OF DEORANE Robert D. Schultz, East Whittier, and Carl L. Randolph,

Jr., Whittier, Califi, assignors to Aerojet-General Corporation, Azusa, Califi, a corporation of Ohio No Drawing. Filed Mar. 10, 1955, Ser. No. 493,568 11 Claims. (Cl. 23-204) This invention relates to a new and improved method of preparing diborane.

Diborane is an excellent high energy fuel. Unfortunately, however, its use has been severely limited due to a lack of a convenient and inexpensive method for preparing it in high yield.

Heretofore, diborane has been prepared by reacting alkali metal borohydrides with hydrogen chloride gas at room temperature or with liquid hydrogen chloride at a temperature of -78 C. These methods are very slow, requiring reaction times of from 16 to 24 hours. In addition, the separation of diborane from the resulting 70% diborane-30% hydrogen chloride azeotropic mixture is very difiicult. It has long been desired as a matter of cost, convenience and commercial feasibility, to produce diborane by using sulfuric acid as the protolyzing acid, preferably at room temperature. Unfortunately, when alkali metal borohydrides were reacted with concentrated sulfuric acid, instead of diborane boron oxides were produced with explosive violence. In an attempt to avoid oxidation, dilute sulfuric acid was substituted in the reaction. Although the borohydride-dilute sulfuric acid reaction was not explosive, hydrolysis occurred producing boric acid instead of diborane.

We have now found that diborane is rapidly and safely produced in good yield by reacting a metal borohydride with concentrated sulfuric or chlorosulfonic acid in the presence of a polycyclic aromatic hydrocarbon, in accordance with the general reaction scheme set forth below:

polycyclic aromatic hydrocarbon wherein M is a metal radical. Due to the non-volatile character of sulfuric acid, an azeotropic mixture of gases is avoided. Although rapid, the reaction proceeds smoothly and gently without sparking, flaming or ex plosions.

As a matter of convenience, alkali and alkaline earth metal borohydrides such as potassium and lithium borohydride are usually employed as starting materials. Commercial grade sulfuric acid, ordinarily containing from about 95% to about 98% by weight pure hydrogen sulfate and about 5% to about 2% by weight water, can be used as the protolyzing acid, if desired. Chlorosulfonic acid decomposes in water and therefore it is preferred that the system be anhydrous when this acid is employed. Mixtures of chlorosulfonic and sulfuric acid may also be used if desired.

Aromatic polycyclic hydrocarbons useful in this invention are naphthalene, anthracene, phenanthrene and the alkylated compounds such as fluorene, a-methylnaphthalene, ,B-methylnaphthalene, 1,3-dimethylanthracene, 2,3-dimethylanthracene, 2,4-dimethylanthracene, 9- ethylanthracene, B methylanthracene, 9,10 dimethylphenanthrene, 1 methylphenanthrene and 3 methylphenanthrene as well as mixtures of such aromatic polycyclic hydrocarbons.

In accordance with the present invention, sulfuric or chlorosulfonic acid containing a polycyclic aromatic hydrocarbon, usually in an amount of from about 2% to about 10% by weight of the acid, is reacted with a metal borohydride and diborane gas is evolved. The preferred method of conducting the reaction is to dissolve Patented June 2, 1964 the aromatic hydrocarbon in the concentrated sulfuric or chlorosulfonic acid, heating if necessary to effect solution. The reaction is usually conducted in a reactor equipped with a gas outlet leading to a low temperature trap. The acid solution is placed in the reactor and finely divided borohydride added with agitation. During the course of the reaction, a gentle evolution of hydrogen and diborane gases takes place. These gases are vented through the gas outlet and collected in the low temperature trap. If desired, the reaction can also be conducted in vacuum or a nitrogen atmosphere as well as in air.

The mechanism by which polycyclic aromatic hydrocarbons modify the sulfuric or chlorosulfonic acid-borohydride reaction has not been definitely established. It is believed, however, that the polycyclic aromatic hydrocarbons react with the protolyzing acid to produce derivatives which reduce the surface tension of the protolyzing acid and form a coating about the borohydride particles. The reactants diffuse into this coating. In addition, bisulfate ions produced by the dissociation of protolyzing acid become concentrated in the coating, thus providing a bulfer in the reaction zone. By diffusing of reactants and buffering of the reaction, the protolysis of the borohydride yielding diborane takes place without accompanying oxidation which produces boron oxides, or hydrolysis which produces boric acid.

To more clearly illustrate this invention, the following examples are presented. It is to be understood, however, that these examples are presented merely as a means of illustration and are not intended to limit the scope of the invention in any way.

EXAMPLE I Naphthalene Moderation Naphthalene in an amount of 3% by weight of the solution was dissolved in concentrated sulfuric acid by heating the mixture to a temperature of C. A 2 cc. amount of the acid solution was placed in a standard test tube and a crystal of sodium borohydride, approximately 2 x 2 /2 x 3 mm. in size, was dropped into the solution. A gentle evolution of gas occurred and. diborane was identified by its characteristic infrared spectrum.

EXAMPLE ]I Naphthalene Moderation Naphthalene in an amount of 5% by weight of the acid mixture was dissolved in sulfuric acid having a purity of 97% Ninety cc. of the acid mixture was placed in a 3-necked flask fitted with a wire stirrer, a gas outlet leading to a low temperature trap, and a dropping funnel. The reactor was swept with nitrogen and the nitrogen gas flow was maintained. Over a period of about 20 minutes, 5 gm. of sodium borohydride was dropped into the solution. During this time the temperature was maintained at 24 C. and the stirrer was operated at about 300 r.p.m. The solution turned from pink to dark green after 3 gm. of sodium borohydride had been added. The reaction proceeded smoothly without sparking, flaming or any explosion, and diborane was recovered in a 58% yield.

EXAMPLE III F luorene Moderation Fluorene in an amount of 5% by weight of the solution was dissolved in concentrated sulfuric acid by heating the mixture to a temperature of 100 C. A 2 cc. amount of the acid solution was placed in a standard test tube and a crystal of sodium borohydride, approximately 2 x 2 /2 x 3 mm. in size, was dropped into the solution. A gentle evolution of gas occurred and di- 3 borane was identified by its characteristic infrared spectrum.

Optimum results are obtained by first dissolving the modifier in the protolyzing acid and subsequently adding the metal borohydride to the solution. This procedure provides'a homogeneous reaction mixture in which maximum protection of the metal borohydride particles from the oxidizing influence of the protolyzing acid is obtained as well as maximum surface area for reaction. It is often desirable to heat the modifier-protolyzing acid mixture to effect solution, however, the temperature at which the diborane producing reaction proceeds is not limited by the expedient. The reactants and products of the reaction are thermally stable and the temperature at which the reaction is conducted is not critical, although as a matter of convenience the reaction is conducted at about room temperature.

We have invented a means of moderating the reaction of borohydrides with sulfuric or chlorosulfonic acid so that diborane is produced safely, rapidly and conveniently. Due to the inexpensiveness and availability of sulfuric acid, ease of running the reaction at ordinary temperatures, and the convenience of recovering diborane from the reaction mixture, the above described method of preparing diborane will find valuable use in the production of this high energy fuel. As well as being useful as a high energy fuel, diborane also finds valuable use in vulcanizing rubber, as disclosed in United States Patent No. 2,558,559.

We claim:

1. A method of producing diborane which comprises reacting a protolyzing acid selected from the group consisting of concentrated sulfuric and concentrated chlorosulfonic acid and mixtures thereof with a metal borohydride selected from the group consisting of alkali and alkaline earth metal borohydrides in the presence of a polycyclic aromatic hydrocarbon selected from the group consisting of naphthalene, anthracene, phenanthrene, fluorene, lower alkylnaphthalene, lower alkylanthracene, lower alkylphenanthrene and mixtures thereof.

2. The method of claim 1 wherein the polycyclic aromatic hydrocarbon is present in an amount of from about 2% to about by weight of the protolyzing acid.

3. The method of claim 1 wherein the metal borohydride is sodium borohydride.

4. The method of claim 1 wherein the metal borohydride is potassium borohydride.

5. The method of claim 1 wherein the metal borohydride is lithium borohydride.

6. The method of claim 1 wherein the polycyclic aromatic hydrocarbon is present in an amount of from about 2% to about 10% by weight of the protolyzing acid.

7. The method of claim 6 wherein said protolyzing acid and polycyclic aromatic hydrocarbon are first mixed together and said metal borohydride then added to the mixture.

8. A method of preparing diborane which comprises reacting a mixture of concentrated sulfuric acid and sodium borohydride in the presence of from about 2% to about 10% naphthalene, by weight of sulfuric acid.

9. A method of preparing diborane which comprises reacting a mixture of concentrated sulfuric acid and sodium borohydride in the presence of from about 2% to about 10% fluorene, by weight of sulfuric acid.

10. A method of producing diborane which comprises reacting a protolyzing acid selected from the group consisting of concentrated sulfuric and concentrated chlorosulfonic acid and mixtures thereof with a metal borohydride selected from the group consisting of the alkali and alkaline earth metal borohydrides in the presence of naphthalene.

11. A method of producing diborane which comprises reacting a protolyzing acid selected from the group consisting of concentrated sulfuric and concentrated chlorosulfonic acid and mixtures thereof with a metal borohydride selected from the group consisting of the alkali and alkaline earth metal borohydrides in the presence of fluorene.

References Cited in the file of this patent UNITED STATES PATENTS 2,513,997 Gibb July 4, 1950 2,543,511 Schlesinger et a1. Feb. 27, 1951 2,880,068 Chiras Mar. 31, 1959 OTHER REFERENCES Hurd: Chemistry of the Hydrides, page 162 (1952), published by John Wiley and Sons, N.Y.C.

Schechter et al.: Boron Hydrides and Related Compounds, page 45, January 8, 1951, declassified January 5, 1954. Dept. of Navy Bureau of Aeronautics.

Wiberg et al.: Zeitschrift fiir Naturforschung, vol. 7b, pages 58-59 (1952).

Richter: Textbook of Organic Chemistry," 2nd Edition, 1943, pages 531, 545-547.

Kilpatrick et al.: J. Am. Chem. Soc, vol. 72, pages 5474 to 5476 (1950). 

1. A METHOD OF PRODUCING DIBORANE WHICH COMPRISES REACTING A PROTOLYZING ACID SELECTED FROM THE GROUP CONSISTING OF CONCENTRATED SULFURIC AND CONCENTRATED CHLOROSULFONIC ACID AND MIXTURES THEREOF WITH A METAL BOROHYDRIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METAL BOROHYDRIDES IN THE PRESENCE OF A POLYCYCLIC AROMATIC HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF NAPHTHALENE, ANTHRACENE, PHENANTHRENE, FLUORENE, LOWER ALKYLNAPHTHALENE, LOWER ALKYLANTHRACENE, LOWER ALKYLPHENANTHRENE ANE MIXTURES THEREOF. 